DNS Transport over TCP - Implementation Requirements

Versions: 00 01 02 03 rfc5966                                           
DNSEXT                                                         R. Bellis
Internet-Draft                                                Nominet UK
Updates: 1035, 1123                                     October 26, 2009
(if approved)
Intended status: Standards Track
Expires: April 29, 2010

                         DNS Transport over TCP

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   This document updates the requirements for the support of the TCP

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   protocol for the transport of DNS traffic.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3

   2.  Terminology used in this document . . . . . . . . . . . . . . . 3

   3.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . . . 3

   4.  Transport Protocol Selection  . . . . . . . . . . . . . . . . . 4

   5.  Dormant Connection Handling . . . . . . . . . . . . . . . . . . 5

   6.  Response re-ordering  . . . . . . . . . . . . . . . . . . . . . 6

   7.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6

   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6

   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 6
     9.1.  Normative References  . . . . . . . . . . . . . . . . . . . 6
     9.2.  Informative References  . . . . . . . . . . . . . . . . . . 7

   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . . . 7

   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 7

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1.  Introduction

   Most DNS [RFC1035] transactions take place over the UDP [RFC0792]
   protocol.  The TCP [RFC0793] protocol is used for zone transfers and
   is supported by many implementations for the transfer of other
   packets which exceed the protocol's original 512 byte packet-size

   Section of [RFC1123] states:

      DNS resolvers and recursive servers MUST support UDP, and SHOULD
      support TCP, for sending (non-zone-transfer) queries.

   However, some implementors have taken the text quoted above to mean
   that TCP support is truly optional for typical DNS operation.

   This document normatively updates the core DNS protocol
   specifications such that (except in very limited circumstances)
   support for the TCP protocol is henceforth REQUIRED.

2.  Terminology used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.  Discussion

   In the absence of EDNS0 (see below) the normal behaviour of any DNS
   server needing to send a UDP response that exceeds that 512 byte
   limit is for the server to truncate the response at the 512 byte
   limit and set the TC flag in the response header.  When the client
   receives such a response it takes the TC flag as notice that it
   should retry over TCP instead.

   RFC 1123 also says:

      ... it is also clear that some new DNS record types defined in the
      future will contain information exceeding the 512 byte limit that
      applies to UDP, and hence will require TCP.  Thus, resolvers and
      name servers should implement TCP services as a backup to UDP
      today, with the knowledge that they will require the TCP service
      in the future.

   Existing deployments of DNSSEC [RFC4033] have shown that truncation
   at the 512 byte boundary is now commonplace.  For example an NXDOMAIN

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   (RCODE == 3) response from a DNSSEC signed zone using NSEC3 [RFC5155]
   is almost invariably longer than 512 bytes.

   Since the original core specifications for DNS were written, the
   Extension Mechanisms for DNS (EDNS0 [RFC2671]) have been introduced.
   These extensions can be used to indicate that the client is prepared
   to receive UDP responses longer than 512 bytes.  An EDNS0 compatible
   server receiving a request from an EDNS0 compatible client may send
   UDP packets up to that client's announced buffer size without

   However, transport of UDP packets which exceed the size of the path
   MTU has been found to be unreliable in some circumstances because of
   IP packet fragmentation.  Many firewalls routinely block fragmented
   IP packets, and some implementations lack the software logic
   necessary to reassemble a fragmented datagram.  Worse still, some
   devices deliberately refuse to handle DNS packets containing EDNS0
   options.  Other issues relating to UDP transport and packet size are
   discussed in [RFC5625].

   The MTU most commonly found in the core of the Internet is around
   1500 bytes, and even that limit is routinely exceeded by DNSSEC
   signed responses.

   The future that was anticipated in RFC 1123 has arrived, and the only
   standardised mechanism which may have resolved the packet size issue
   has been found inadequate.

4.  Transport Protocol Selection

   All DNS implementations MUST support both UDP and TCP transport
   protocols, except as set out below.

   On a case by case basis, authoritative DNS server operators MAY elect
   to disable DNS transport over TCP if all of the following conditions
   are satisfied:

   o  the server is authoritative only
   o  the server does not support AXFR
   o  all requests and responses are guaranteed to be <= 512 bytes

   A general purpose stub resolver implementation (e.g. an operating
   system's DNS resolution library) MUST support TCP since to do
   otherwise would limit its interoperability with its own clients and
   with upstream servers.

   A proprietary stub resolver implementation MAY omit support for TCP

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   if it is operating in an environment where truncation can never
   occur, or if it is prepared to accept a DNS lookup failure should
   truncation occur.

   A recursive resolver or forwarder MUST support TCP so that it does
   not prevent long responses from a TCP-capable server from reaching
   its TCP-capable clients.

   Regarding the choice of when to use UDP or TCP, RFC 1123 says:

      ... a DNS resolver or server that is sending a non-zone-transfer
      query MUST send a UDP query first.

   That requirement is hereby relaxed.  A resolver SHOULD send a UDP
   query first, but MAY elect to send a TCP query instead if it has good
   reason to expect the response would be truncated if it were sent over
   UDP (with or without EDNS0) or for other operational reasons.

5.  Dormant Connection Handling

   Section 4.2.2 of [RFC1035] says:

      If the server needs to close a dormant connection to reclaim
      resources, it should wait until the connection has been idle for a
      period on the order of two minutes.

   Other more modern protocols (e.g.  HTTP [RFC2616]) have support for
   persistent TCP connections and operational experience has shown that
   long timeouts can easily cause resource exhaustion and poor response
   under heavy load.  Intentionally opening many connections and leaving
   them dormant can trivially create a "denial of service" attack.

   This document therefore RECOMMENDS that the idle period should be of
   the order of TBD seconds.

   Servers MAY allow dormant connections to remain open for longer
   periods, but for the avoidance of doubt persistent DNS connections
   should generally be considered to be as much for the server's benefit
   as for the client's.  Therefore if the server needs to unilaterally
   close a dormant TCP connection it MUST be free to do so whenever

   Further recommendations for the tuning of TCP parameters to allow
   higher throughput or improved resiliency against denial of service
   attacks are (currently) outside the scope of this document.

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6.  Response re-ordering

   RFC 1035 is ambiguous on the question of whether TCP queries may be
   re-ordered - the only relevant text is in Section 4.2.1 which relates
   to UDP:

      Queries or their responses may be reordered by the network, or by
      processing in name servers, so resolvers should not depend on them
      being returned in order.

   For the avoidance of future doubt, this requirement is clarified.
   Client resolvers MUST be able to process responses which arrive in a
   different order to that in which the requests were sent, regardless
   of the transport protocol in use.

7.  Security Considerations

   Some DNS server operators have expressed concern that wider use of
   DNS over TCP will expose them to a higher risk of "denial of service"

   Many large authoritative DNS operators including all but one of the
   root servers and the vast majority of TLDs already support TCP and
   attacks against them are infrequent and very rarely successful.

   Operators of recursive servers should ensure that they only accept
   connections from expected clients, and do not accept them from
   unknown sources.  In the case of UDP traffic this will protect
   against reflector attacks [RFC5358] and in the case of TCP traffic it
   will prevent an unknown client from exhausting the server's limits on
   the number of concurrent connections.

8.  IANA Considerations

   This document requests no IANA actions.

9.  References

9.1.  Normative References

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, September 1981.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, September 1981.

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   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [RFC1123]  Braden, R., "Requirements for Internet Hosts - Application
              and Support", STD 3, RFC 1123, October 1989.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2671]  Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
              RFC 2671, August 1999.

9.2.  Informative References

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, March 2005.

   [RFC5155]  Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
              Security (DNSSEC) Hashed Authenticated Denial of
              Existence", RFC 5155, March 2008.

   [RFC5358]  Damas, J. and F. Neves, "Preventing Use of Recursive
              Nameservers in Reflector Attacks", BCP 140, RFC 5358,
              October 2008.

   [RFC5625]  Bellis, R., "DNS Proxy Implementation Guidelines",
              BCP 152, RFC 5625, August 2009.

Appendix A.  Change Log

   NB: to be removed by the RFC Editor before publication.

      Addition of response ordering section
      Various minor editorial changes from WG reviewers

      Initial draft

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Author's Address

   Ray Bellis
   Nominet UK
   Edmund Halley Road
   Oxford  OX4 4DQ
   United Kingdom

   Phone: +44 1865 332211
   Email: ray.bellis@nominet.org.uk
   URI:   http://www.nominet.org.uk/

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